(1) Field of the Invention
The present invention relates to methods and apparatuses for stabilizing vertebrae when fusing the vertebrae.
(2) Description of the Related Art
Fusion of vertebrae is often necessary to relieve debilitating pain or correct a deformity. When vertebrae are fused, e.g., with bone grafts, graft extenders, or interbody spacers such as interbody cages or boxes (collectively termed “grafts” herein), it is desirable to stabilize the fused vertebrae using an apparatus such as a plate to fixate one cervical vertebra to another to promote fusion across motion segments.
There are various known apparatuses useful for stabilizing vertebrae when the vertebrae are fused. See, e.g. U.S. Pat. Nos. 4,743,260; 5,603,703; 5,458,641; 5,827,328; and 6,080,193.
The known apparatuses are generally useful to prevent horizontal displacement of the two grafted vertebrae. They also can prevent excessive compression of the two vertebrae, which can lead to a weak fusion or even collapse of the graft. However, the apparatuses can also lead to stress shielding, in which fusion of the vertebrae to the grafted bone is impeded or prevented entirely because the apparatus prevents adequate contact between the vertebra and the graft. Resorption of the bone graft can exacerbate this problem. It is well known in the art that some subsidence, or settling, between the vertebrae at the graft is advantageous to quickly forming a strong fusion. The subsidence increases bone to bone contact, which is well known to enhance bone fusion, as predicted by Wolff's law, by enhancing physiological processes involved in bone remodeling (Kowalski et al., 2001, Neurol. Focus 10 (4) Article 2).
The problem of stress shielding is partially addressed in U.S. Pat. No. 5,843,082, which provides an apparatus having a plate on each vertebra, and a pair of longitudinal rods which interconnect the plates. The plates can slide along the rods vertically, which allows for subsidence between the vertebrae. There is no suggestion therein that the amount of subsidence could or should be controlled using that apparatus. Thus, while the apparatus in the '082 patent does not cause stress shielding, it also does not prevent excessive subsidence.
It is therefore an object of the present invention to provide improved apparatuses and methods for promoting optimal fusion at a graft, particularly at cervical vertebrae.
It is a further object of the invention to provide improved apparatuses and methods for stabilizing the fusion that minimize stress shielding.
It is an additional object of the invention to provide apparatuses and methods for minimizing stress shielding as well as excessive subsidence at vertebral fusions, thus promoting optimal fusion at the graft.
It is a still further object of the invention to provide an apparatus that accomplishes the above objects yet is compact in size and utilizes a minimum amount of parts.